In addition to capturing Mesraba, government forces have also encircled the towns of Douma and Harasta, deep inside the enclave.

Aboud, the activist inside Douma, said many people were afraid.

"They are afraid of the government advance because they know that the regime is spiteful of Douma and Eastern Ghouta," said Aboud.

Syrian state media says government forces now control 51 percent of Eastern Ghouta.

The Syrian government's offensive follows a pattern of previous assaults on opposition strongholds, deploying massive air power and tight sieges to force rebel fighters to accept "evacuation" deals.

These involve rebels surrendering territory in exchange for safe passage to opposition areas in northwest Syria, along with their families and other civilians who do not want to come back under Assad's rule.

On Friday, a number of rebel fighters were evacuated from Eastern Ghouta.

Jaish al-Islam, one of the main rebel groups in Eastern Ghouta, announced it had agreed to the evacuation of several Hay'et Tahrir al-Sham fighters - previously part of the al-Qaeda-linked al-Nusra Front - who were detained by the group in Eastern Ghouta.

According to Syrian state media, 13 fighters were evacuated with their families through the al-Wafeedin passage and bussed to Idlib province.

Marwan Kabalan, a Doha-based Syrian analyst, says the deal is part of Assad's strategy to divide and conquer.

"He's isolating the two big cities [Douma and Harasta] in Eastern Ghouta - he's weakening them and he's supporting this military approach through a political strategy - separating them on the ground while trying to strike a deal with Jaish al-Islam," Kabalan said.

'Verge of major disaster'

The United Nations' refugee agency representative to Syria, Sajjad Malik, entered the town of Douma in Eastern Ghouta on Monday accompanying an aid convoy.

When asked what he saw in Eastern Ghouta, Malik said the area was "on the verge of a major disaster".

"You walk in that town and you see destruction, you see displacement. There are dead bodies still in destroyed buildings. The smell is very strong in those areas," he said.

"There are people who are living in overcrowded basements. People coming out of those basements when we were there - it's hard to describe them. Their pale skin, you could see rashes on their skin, you can see that they're stunted."